The commonest flowmeters for a fluid such as a gas or a liquid comprise mechanical moving parts, such as spinner or membrane flowmeters. However there are also flowmeters using fluid oscillators which do not have moving parts but which measure the vibrations of a fluid within an oscillation chamber. These oscillators can be very small and of very simple construction. Their reliability is thus very good. Moreover they provide a frequency signal whose frequency is proportional to the speed of the fluid entering the oscillation chamber. This signal can easily be converted to a digital signal. This property is particularly advantageous for reading meters remotely.
There are many forms of fluid oscillator which produce vibrations in a fluid using various effects. Vortex flowmeters use an obstruction positioned in a duct to create vortices in a fluid flowing in the duct. For an obstacle of given geometry, the frequency of detachment of the vortices is proportional to the speed of flow of the fluid. An oscillator of this kind is described in U.S. Pat. No. 4,085,615. Coanda effect oscillators use the tendency of a fluid jet to follow the contours of a wall when the jet is discharged near to the wall, even if the contour of the wall moves away from the discharge axis of the jet. A fluid oscillator of this type is described in U.S. Pat. No. 4,550,614, in which a fluid jet is discharged into a chamber with two lateral walls disposed symmetrically in the chamber relative to the discharge axis of the jet, the jet oscillating from one wall to the other. There are also other kinds of fluid oscillator which use the natural oscillation of a fluid jet emerging from a nozzle or which use other means to cause a jet to oscillate or to create vortices. The term "fluid oscillator" applies herein to any oscillator which produces fluid vibrations representative of the speed of the fluid.
The detector means, such as a pair of pressure sensors or thermal sensors disposed at different locations in the oscillation chamber, detect the fluid oscillations in the chamber. The detector means supply an alternating signal having a frequency proportional to the speed of the fluid entering the oscillation chamber. Ideally, the mean amplitude of the alternating signal should remain constant. Unfortunately asymmetries in the system, for example arising from the positioning of the sensors, can cause the mean amplitude to change with changes in frequency of the signal, sometimes very significantly. This problem arises in particular with vortex oscillators and flowmeters used in the field of industrial metering, for example in an industrial gas meter, in which the amounts and speed of the fluid are larger than those used in the field of domestic or commercial metering. This strong modulation of the signal creates problems in measuring the frequency and processing the signal to obtain the speed of flow of the fluid. Conventional techniques which compensate for the zero offset of the signal, for example by using an integrating circuit, are generally effective for slow changes in amplitude but they are not effective for rapid variations. Moreover, in the field of metering fluids in which an electric battery is used in the signal processing circuit, it is necessary for current consumption to be minimized, to prolong the life of the battery. Conventional circuits for processing and converting signals with large amplitude modulation are relatively complex and consume a lot of current. The signal supplied by the detector means can sometimes be of small amplitude and it is not easy to make use of such a signal when using a battery and consuming little power.
A circuit is known from U.S. Pat. No. 3,982,434 which reduces the low frequency components of the flow signals detected by a thermal sensor in an eddy fluid meter and extracts the high frequency components from this signal. That circuit comprises an inverting amplifier, a peak detector, a summing amplifier, and a trigger amplifier. The peak detector detects positive and negative peaks in the flow signal detected by the thermal sensor and provides a signal whose frequency corresponds to the rate of flow of the fluid.
Nevertheless, that circuit operates with relatively high supply voltages of 12V and 24V and it is not suited to battery operation, nor to small power consumption. Furthermore the circuit described in U.S. Pat. No. 3,982,434 is somewhat complex.